Add E009 update-from-without-join (T-SQL implicit-join Cartesian risk)

[Pawansingh3889]

Closes #42.

T-SQL accepts UPDATE customers SET status = o.status FROM customers c, orders o WHERE c.customer_id = o.customer_id. The comma form is a legacy implicit join: get the WHERE wrong (or omit it) and every row in the target table is updated against the Cartesian product. Silent data corruption, no syntax error.

Reported as severity = error rather than warning, because the failure mode is a write that touches every row of a table, not a read. The sister rule S001 covers SELECT FROM comma-joins as a warning.

Implementation mirrors the post-fix S001 from #46. Walk from each UPDATE keyword to the next FROM, then forward through the FROM clause tracking parenthesis depth, stop at WHERE / GROUP BY / ORDER BY / HAVING / LIMIT / explicit JOIN / UNION / EXCEPT / INTERSECT, and flag the first depth-0 comma. The strip_strings_and_comments helper that S001 introduced has been lifted to sql_guard.rules.base (first commit of this PR) so E009 reuses it without duplicating the regexes.

Deliberate suppression cases:

UPDATE c SET tag = sub.tag FROM customers c, LATERAL (SELECT tag FROM tags WHERE customer_id = c.id LIMIT 1) sub;
UPDATE customers SET status = o.status FROM orders o WHERE customers.id = o.customer_id;
UPDATE customers SET status = 'active' WHERE id = 1;
UPDATE c SET c.status = o.status FROM customers c INNER JOIN orders o ON c.customer_id = o.customer_id;

LATERAL after a comma is a real Snowflake / Postgres lateral join. Postgres canonical UPDATE FROM with a single table is the recommended form. Plain UPDATE with no FROM is fine. The recommended fix from the rule message must not flag itself.

Tests: 7 new methods on TestErrorRules covering the issue example via the fixture, the recommended fix, Postgres single-table UPDATE FROM, LATERAL after comma, plain UPDATE without FROM, three-table comma join, and multi-line comma join. Plus an E009 line added to tests/fixtures/errors.sql.

Four commits on this branch. The first is a behaviour-preserving refactor lifting strip_strings_and_comments from structural.py to base.py so E009 reuses it. The next adds the rule in errors.py and registers it; registry counts move to 40 total and 11 errors. The third adds tests and the fixture line. The fourth documents the rule in CHANGELOG under [Unreleased] Added.

Full suite: 226 pass, 1 skipped. No regressions. Commits signed (ED25519).